Background: The early and accurate diagnosis of obstructive uropathy in the newborn, prevents secondary complications and kidney damage. Aim: To study the usefulness of Tc99M MAG3 diuretic renogram in newborns with hydronephrosis. Material and methods: Forty newborns, aged 1 to 30 days, with hydronephrosis, were studied. A Tc99M MAG3 diuretic renogram (DR) was done and its results were compared with clinical features and other imaging studies. Each kidney and its ureter, were considered a renal unit. Results: Seventy six renal units were evaluated. Twenty six were normal on prenatal ultrasound examination and DR. In 11 of the 50 renal units with hydronephrosis, renal function was impaired. Thus, it was impossible to obtain an excretory curve. In 17 of the 39 remaining renal units, the absence of obstructive uropathy was demonstrated clinically. In 16 of these, the DR showed absence of obstruction. In 20 of 21 renal units with confirmed obstructive uropathy, DR showed an obstructive pattern. Conclusions: In newborns, there is an adequate Tc99M MAG3 uptake and diuretic response. Thus, DR becomes a good functional assessment method in newborns with hydronephrosis (Rev Méd Chile 2003; 131: 251-8).